Multiplexed hydraulic control systems use a hydraulic multiplexer to multiplex or otherwise distribute the controlled output flow of a hydraulic control to multiple second stage valves and/or actuators. Significant cost and weight advantages exist in using hydraulically multiplexed control systems as opposed to separate hydraulic control systems because a dedicated hydraulic control for each actuator is not necessary. However, to realize the cost and weight advantages, it is often necessary for hydraulically multiplexed systems to match or exceed the performance that is achieved by non-multiplexed hydraulic control systems. To furnish similar performance as non-multiplexed systems, multiplexed control systems have needed to supply a sufficient amount of fluid flow to generate adequate force and/or sufficient rate of movement in the actuator. In some applications, amplification mechanisms in the form of second stage valves have been used to further increase the fluid flow and thereby the performance as shown in Leeson et al., U.S. Pat. No. 4,984,505, McLevige, et al., U.S. Pat. No. 5,048,394, Wardle, et al., U.S. Pat. No. 5,054,369, Wardle, et al., U.S. Pat. No. 5,081,903, and Wardle, et al., U.S. Pat. No. 5,088,383, the disclosures of which are hereby incorporated by reference.
The action of each second stage valve is generally that of a sample and hold device. Each second stage valve has a position which is periodically or sequentially updated from the hydraulic control. The position of each second stage valve is determined by the time integral of fluid flow from the hydraulic control. The position of the second stage valve determines a continuous fluid flow rate to the actuator. The second stage valve position is held until the next update from the hydraulic control.
One of the significant and exemplary applications of multiplexed hydraulic control systems is in aircraft systems where hydraulic controls are provided for adjusting the mechanical variables of gas turbine engines. Such hydraulic control systems using intermediate integrating second stage valves may be used when a large flow capacity is required, such as control of variable stator vanes. Variable stator vanes control the airstream entering the compressor blades in a gas turbine engine to improve engine performance.
A problem with such hydraulic control systems using integrating second stage valves occurs during electrical power loss to the hydraulic control. Under power loss conditions, it may be desired that certain actuators slew in a predetermined direction at a controlled rate. For example, in gas turbine engine systems it is desired that the actuator controlling the variable stator vanes slew in a closed direction at a controlled rate upon electrical power loss. In systems where the output flow of a hydraulic control is directly connected to an actuator, the hydraulic control may have a null bias which corresponds to a predetermined output flow to move the actuator in a predetermined direction at a predetermined rate upon power loss. However, in hydraulic control systems utilizing integrating sample and hold second stage valves, a predetermined output flow from a hydraulic control causes each actuator to move at an increasing rate each time its corresponding second stage valve is updated. This causes the actuators to slew uncontrolled at maximum velocity. Such uncontrolled maximum velocity slew is unacceptable in many situations, particularly for actuators controlling certain critical functions such as the variable stator vanes in gas turbine engines.